Lifted by the collision of the Indian tectonic plate with the Eurasian Plate,[3] the Himalayan range runs northwest to southeast in a 2,400 kilometres (1,500 mi) long arc. The range varies in width from 400 kilometres (250 mi) in the west to 150 kilometres (93 mi) in the east. Besides the Greater Himalayas, there are several parallel lower ranges. The southernmost of these, located along the northern edge of the Indian plains and reaching about a thousand meters in altitude, are called the Sivalik Hills. Further north is a higher range, reaching two to three thousand meters, known as the Lower Himalayan Range.

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The flora and fauna of the Himalayas vary with climate, rainfall, altitude, and soils. The climate ranges from tropical at the base of the mountains to permanent ice and snow at the highest elevations. The amount of yearly rainfall increases from west to east along the southern front of the range. This diversity of altitude, rainfall and soil conditions combined with the very high snow line supports a variety of distinct plant and animal communities. For example the extremes of high altitude (low atmospheric pressure) combined with extreme cold allow extremophile organisms to survive.[4]

The unique floral and faunal wealth of the Himalayas is undergoing structural and compositional changes due to climate change. The increase in temperature may shift various species to higher elevations. The oak forest is being invaded by pine forests in the Garhwal Himalayan region. There are reports of early flowering and fruiting in some tree species, especially rhododendron, apple and box myrtle. The highest known tree species in the Himalayas is Juniperus tibetica located at 4,900 metres (16,080 ft) in Southeastern Tibet.[5]

During the Upper Cretaceous, about 70 million years ago, the north-moving Indo-Australian Plate was moving at about 15 cm per year. About 50 million years ago this fast moving Indo-Australian plate had completely closed the Tethys Ocean, the existence of which has been determined by sedimentary rocks settled on the ocean floor, and the volcanoes that fringed its edges. Since both plates were composed of low density continental crust, they were thrust faulted and folded into mountain ranges rather than subducting into the mantle along an oceanic trench.[3] An often-cited fact used to illustrate this process is that the summit of Mount Everest is made of marine limestone from this ancient ocean.[6]

Today, the Indo-Australian plate continues to be driven horizontally below the Tibetan Plateau, which forces the plateau to continue to move upwards. The Indo-Australian plate is still moving at 67 mm per year, and over the next 10 million years it will travel about 1,500 km into Asia. About 20 mm per year of the India-Asia convergence is absorbed by thrusting along the Himalaya southern front. This leads to the Himalayas rising by about 5 mm per year, making them geologically active. The movement of the Indian plate into the Asian plate also makes this region seismically active, leading to earthquakes from time to time.

During the last ice age, there was a connected ice stream of glaciers between Kangchenjunga in the east and Nanga Parbat in the west.[7][8] In the west, the glaciers joined with the ice stream network in the Karakoram, and in the north, joined with the former Tibetan inland ice. To the south, outflow glaciers came to an end below an elevation of 1,000–2,000 metres (3,300–6,600 ft).[7][9] While the current valley glaciers of the Himalaya reach at most 20 to 32 kilometres (12 to 20 mi) in length, several of the main valley glaciers were 60 to 112 kilometres (37 to 70 mi) long during the ice age.[7] The glacier snowline (the altitude where accumulation and ablation of a glacier are balanced) was about 1,400–1,660 metres (4,590–5,450 ft) lower than it is today. Thus, the climate was at least 7.0 to 8.3 °C (12.6 to 14.9 °F) colder than it is today.[10]

Owing to the mountains' latitude near the Tropic of Cancer, the permanent snow line is among the highest in the world at typically around 5,500 metres (18,000 ft).[13] In contrast, equatorial mountains in New Guinea, the Rwenzoris and Colombia have a snow line some 900 metres (2,950 ft) lower.[14] The higher regions of the Himalayas are snowbound throughout the year, in spite of their proximity to the tropics, and they form the sources of several large perennial rivers, most of which combine into two large river systems:

The western rivers combine into the Indus Basin, of which the Indus River is the largest. The Indus begins in Tibet at the confluence of Sengge and Gar rivers and flows southwest through India and then through Pakistan to the Arabian Sea. It is fed by the Jhelum, the Chenab, the Ravi, the Beas, and the Sutlej rivers, among others.

The easternmost Himalayan rivers feed the Ayeyarwady River, which originates in eastern Tibet and flows south through Myanmar to drain into the Andaman Sea.

The Salween, Mekong, Yangtze and Huang He (Yellow River) all originate from parts of the Tibetan Plateau that are geologically distinct from the Himalaya mountains, and are therefore not considered true Himalayan rivers. Some geologists refer to all the rivers collectively as the circum-Himalayan rivers.[16] In recent years, scientists have monitored a notable increase in the rate of glacier retreat across the region as a result of global climate change.[17] For example, Glacial lakes have been forming rapidly on the surface of the debris-covered glaciers in the Bhutan Himalaya during the last few decades. Although the effect of this will not be known for many years, it potentially could mean disaster for the hundreds of millions of people who rely on the glaciers to feed the rivers of northern India during the dry seasons.[18] Some of the lakes present a danger of a glacial lake outburst flood. The Tsho Rolpa glacier lake in the Rolwaling Valley is rated as the most dangerous in Nepal.[19][20]

The Himalayas have a profound effect on the climate of the Indian subcontinent and the Tibetan Plateau. They prevent frigid, dry winds from blowing south into the subcontinent, which keeps South Asia much warmer than corresponding temperate regions in the other continents. It also forms a barrier for the monsoon winds, keeping them from traveling northwards, and causing heavy rainfall in the Terai region. The Himalayas are also believed to play an important part in the formation of Central Asian deserts, such as the Taklamakan and Gobi.[22]

Several places in the Himalayas are of religious significance in Hinduism, Jainism, Sikhism, and Buddhism. A notable example of a religious site is Paro Taktsang, where Padmasambhava is said to have founded Buddhism in Bhutan.[24]